This invention relates generally to multi-wavelength polarisation monitors and more particularly to a multi-wavelength polarisation monitor resulting in a very compact device manufactured at relatively low cost.
The rapid adoption of the Internet has created a need for high-speed optical networks. High-speed optical networks use wavelength-division-multiplexed (WDM) optical fibre systems characterised by the simultaneous transmission through many different communication channels over different wavelengths within a single optical fibre. For example, communication channels are over wavelengths typically within the 1530-1565 nanometer (nm) range, and separated by multiples of 100 Gigahertz (GHz), i.e., approximately 0.8 nm.
An optical monitor is a critical component in optical networks. Within such systems, the ability to efficiently monitor the performance of the system for the different wavelengths of light travelling through a single fibre is highly desirable during installation and testing as well as normal operation. Typically optical monitors accompany multi-wavelength fibre optic amplifiers, such as erbium-doped fibre amplifiers, and are used for providing feedback. They are also useful in ensuring maintained coupling efficiencies and in test equipment for optical communication equipment. In particular, monitoring of the polarisation of the transmitted light is highly desirable. Polarised light is is highly sensitive to coupling related losses as well as other polarisation dependent loss (PDL). Performance of numerous components of an optical network is either affected by the polarisation of the transmitted light or components must be designed to meet strict PDL specifications.
Conventional optical monitors for multi-wavelength fibre optic communication needs come in a variety of different configurations. In U.S. Pat. No. 6,078,709 an optical monitor is demonstrated. The monitor has a grating in a fibre for deflecting a portion of light propagating within the fibre out of the fibre. This deflected portion is then detected with a detector. The grating disclosed achieves multi-wavelength behaviour through application of heat in order to vary its frequency response. As the grating expands, the wavelength of light that it deflects changes. By controlling the temperature precisely a known wavelength is monitored precisely. This design has the benefit of being able to monitor any wavelength within a given range, however it is only capable of supporting monitoring of one wavelength channel at a time. Further, a precise heating element is needed to provide adequate performance.
Another common method for performing multi-wavelength monitoring is to tap off some of a light signal and to use a wavelength dispersing element in order to separate the light such that light within each channel is incident upon a different detector from a plurality of optical detectors. This device has the advantage that all of the wavelengths are monitored at once. Unfortunately, such a device is often produced in bulk optics and is hampered by the above mentioned manufacturing bottlenecks. When manufactured with integrated optical technology, the resulting demultiplexer has many output ports for coupling to fibres and, as such is extremely difficult to manufacture.
Conventional polarisation monitors use a polarisation splitter for splitting incoming light into its principal states of polarisation as used, for example, in the device disclosed in U.S. Pat. No. 6,130,766, which are then separately sensed. However, a polarisation splitter tends to be difficult to build especially to the precise tolerances necessary for a multi-wavelength monitor.
In order to overcome these and other shortcomings of the prior art it is an object of the invention to provide a multi-wavelength polarisation monitor absent a need for extremely precise optical components.
It is further an object of the invention to provide a multi-wavelength polarisation monitor of increased precision by determining the polarisation of light within each of a plurality of wavelength channels.
The invention demonstrates a relatively simple way and device for monitoring polarisation in multi-wavelength applications. The polarisation monitor allows manufacture of a very compact device at relatively low cost.
In accordance with the invention, there is provided a method for monitoring the polarisation of a multi-wavelength light signal in an optical network comprising the steps of:
beam splitting the light signal into two portions using a beam splitter having known optical characteristics, wherein the polarization of each of the two portions comprises both a TE and a TM component;
demultiplexing the first signal portion of the two separated signal portions into a first plurality of signals within each of predetermined wavelength ranges corresponding to known channels using a first demultiplexer having known polarisation-dependent characteristics;
detecting the light intensity of each of the first plurality of signals and providing a first plurality of electrical signals in dependence thereupon;
demultiplexing the second signal portion of the two separated signal portions into a second plurality of signals within each of the predetermined wavelength ranges corresponding to the known channels using a second demultiplexer having a known second PDL;
detecting the light intensity of each of the second plurality of signals and providing a second plurality of electrical signals in dependence thereupon;
converting the first and second plurality of electrical signals into a first and second plurality of digital data; and,
using a processor, determining the polarisation of the light signal of each wavelength channel in dependence upon the first and second plurality of digital data and based on the known optical characteristics and the first and second PDL.
In accordance with another embodiment of the invention, there is provided a method for monitoring the polarisation of a multi-wavelength light signal in a optical network comprising the steps of:
beam splitting the light signal into to portions using a beam splitter having known optical characteristics, wherein the polarization of each of the two portions comprises both a TE and a TM component;
disperisng the first signal portion of the two separated signal portions onto a first detector array using a first dispersive element having a known first PDL;
detecting the light intesity incident upon each detector element of the first detector array and providing a first plurality of electrical signals in dependence thereupon;
dispersing the second signal portion of the two separated signal portions onto a second detector array using a second disperive element having a known second PDL;
detecting the light intensity incident upon each detector element of the second detector array and providing a second plurality of electrical signals in dependence thereupon;
converting the first and second plurality of electrical signals into a first and second plurality of digital data; and,
using a processor, determining the polarisation of the light signal of each wavelength channel in dependence upon the first and second plurality of digital data and based on the known optical characteristics and the first and second PDL.
In accordance with another aspect of the invention, there is provided a multi-wavelength polarisation monitor comprising:
and input port of receiving a light signal;
a beam splitter having known optical characteristics for beam splitting the light signal into two portions, wherein the polarization of each of the two portions comprises both a TE and a TM component;
a first demultiplexer having a known first PDL for demultiplexing the first signal portion of the two separated signal portions into a first plurality of signals corresponding to wavelength channels;
a first plurality of detectors for detecting the light intensity of each of the first plurality of signals and providing a first plurality of electrical signals in dependence thereupon;
a second demultiplexer having a known second PDL for demultiplexing the second signal portion of the two separated signal portions into a second plurality of signals corresponding to wavelength channels;
a second plurality of detectors for detecting the light intensity of each of the second plurality of signals and providing a second plurality of electrical signals in dependence thereupon;
an A/D converter for converting the first and second plurality of electrical signals into a first and second plurality of digital data; and,
a processor for determining the polarisation of the light signal of each wavelength channel in dependence upon the first and second plurality of digital data and based on the known optical characteristics and the first and second PDL.
In accordance with the invention, there is further provided a multi-wavelength polarisation monitor comprising:
an input port for receiving a light signal;
a beam splitter having known optical characteristics for beam splitting the light signal into a first and second signal portions, wherein the polarization of each of the two portions comprises both a TE and a TM component;
a first dispersive element having a known first PDL for dispersing the first signal portion onto a first detector array, the light dispersed other than as channelised data within known channels;
a first detector array having more detector elements than a number of known channels for detecting the light intensity incident upon each detector element of the first detector array and providing a first plurality of electrical signals in dependence thereupon;
a second dispersive element having a known second PDL for dispersing the second signal portion onto a second detector array, the light dispersed other than as channelised data within the known channels;
a second detector array having more detector elements than a number of known channels for detecting the light intensity incident upon each detector element of the second detector array and providing a second plurality of electrical signals in dependence thereupon;
an A/D converter for converting the first and second plurality of electrical signals into a first and second plurality of digital data; and,
a processor for performing spectral augmentation on the first and second plurality of digital data and for determining the polarisation of the light signal within each of the predetermined wavelength ranges corresponding to the known channels in dependence upon the spectrally augmented first and second plurality of digital data and based on the known optical characteristics and the first and second PDL.
In accordance with the invention there is yet further provided a multi-wavelength polarisation monitor comprising:
an input port for receiving a light signal;
a dispersive element having a known PDL for dispersing the signal onto a detector array, the light dispersed other than as channelised data within known channels;
a detector array having more detector elements than a number of known channels for detecting the light intensity incident upon each detector element of the detector array and providing a plurality of electrical signals in dependence thereupon;
an A/D converter for converting the plurality of electrical signals into a plurality of first digital data;
a digital port for receiving second digital data; and,
a processor for performing spectral augmentation on the plurality of first data and for determining the polarisation of the light signal within each of the predetermined wavelength ranges corresponding to the known channels in dependence upon the spectrally augmented plurality of first digital data and the received second digital data and based on the known PDL of the dispersive element.